The spinal column is a highly complex system of bones and connective tissues that provides support for the body and protects the delicate spinal flexible connecting member and nerves. The spinal column includes a series of vertebrae stacked one on top of the other, each vertebral body including an inner or central portion of relatively weak cancellous bone and an outer portion of relatively strong cortical bone. Situated between each vertebral body is an intervertebral disc that cushions and dampens compressive forces experienced by the spinal column. A vertebral canal containing the spinal cord and nerves is located posterior to the vertebral bodies. In spite of the complexities, the spine is a highly flexible structure, capable of a high degree of curvature and twist in nearly every direction. For example, the kinematics of the spine normally includes flexion, extension, rotation and lateral bending.
There are many types of spinal column disorders including scoliosis (abnormal lateral curvature of the spine), kyphosis (abnormal forward curvature of the spine, usually in the thoracic spine), excess lordosis (abnormal backward curvature of the spine, usually in the lumbar spine), spondylolisthesis (forward displacement of one vertebra over another, usually in a lumbar or cervical spine) and other disorders caused by abnormalities, disease, or trauma, such as ruptured or slipped discs, degenerative disc disease, fractured vertebra, and the like. Patients that suffer from such conditions usually experience extreme and debilitating pain as well as diminished range of motion and nerve function. These spinal disorders may also threaten the critical elements of the nervous system housed within the spinal column.
One of the most common methods for treating spinal disorders is to immobilize a portion of the spine to allow treatment. Traditionally, immobilization has been accomplished by rigid stabilization. For example, in a conventional spinal fusion procedure, a rigid fixation rod is installed between pedicle screws secured to adjacent vertebrae. The fixation rod cooperates with the screws to immobilize the two vertebrae relative to each other so that fusion may occur. Fusion treatments using rigid stabilization, however, do have some disadvantages. For example, because the immobilized portion of the spine has reduced mobility, additional stresses are transferred to other portions of the spine neighboring or nearby the fused vertebrae. Fusion is also an irreversible procedure.
More recently, dynamic stabilization has been used in spinal treatment procedures. Dynamic stabilization does not result in complete spinal fusion, but instead permits enhanced mobility of the spine while also providing sufficient stabilization to effect treatment. One example of a dynamic stabilization system is the Dynesys® system available from Zimmer Spine, Inc. of Edina, Minn. Such dynamic stabilization systems typically include a flexible spacer positioned between pedicle screws installed in adjacent vertebrae of the spine. Once the spacer is positioned between the pedicle screws, a flexible cord is threaded through a channel in the spacer. The flexible cord is also secured to the pedicle screws by a retainer and set screw, thereby retaining the spacer between the pedicle screws while cooperating with the spacer to permit mobility of the spine.
The dynamic stabilization systems described above and others are naturally installed in a patient during a surgical procedure. Patient recovery from such surgical procedures is greatly enhanced if the tissue, muscle and other parts of the patient that are displaced and affected by the surgery are minimized, including the size and severity of the required incisions. For example, the cord can be inserted through a “puncture hole” or “access channel” used to implant one of the pedicle screws and then advanced to its installed position between the pedicle screws. Due to its flexible nature, however, the cord can be difficult to maneuver through the tissue. Additional tools are often required to accomplish this positioning. As a result, the access channels for the pedicle screws must be made large enough to accommodate the tools and any manipulation required. Increasing the size of the access channels increases the disruption of muscle tissue, which should be minimized to reduce scarring and promote faster recovery times. Therefore, systems and methods that further reduce the amount of disruption to the muscle tissue are highly desirable.